This study uses 9 years of longitudinal, student-level data from the Los Angeles Unified School District to provide updated, empirically-based estimates of the time necessary for English learners (ELs) to become reclassified as proficient in English, as well as factors associated with variation in time to reclassification. To illustrate how different aspects of proficiency develop, estimates of the time necessary for ELs to attain six separate reclassification criteria are provided. Findings corroborate prior cross-sectional research suggesting that the development of full proficiency in a second language typically takes 4 to 7 years. However, after 9 years in the district, approximately one-fourth of students had not been reclassified. There appears to be a reclassification window during the upper elementary grades, and students not reclassified by this point in time become less likely ever to do so. Findings illustrate the crucial role that students’ initial academic language proficiencies, both in English and their primary language, play in their likelihood of reclassification. This work has implications for the design of next-generation assessment and accountability systems, as well as for instructional practices.